The present invention relates to automatic, dynamic compression/expansion systems, particularly for use in high fidelity sound reproduction equipment.
It is known that the quality of the reproduction of audio frequency performances can be improved by increasing the signal-to-noise ratio by means of a compression-expansion system known as a compander system, in which the useful signals are compressed in amplitude before entering a transmission path subject to interference, e.g. a line or a recording tape, and are expanded after transmission.
Known compander systems employing frequency independent dynamic regulation suffer from the drawback that under certain operating conditions a low frequency, high amplitude sound signal will be unable to cover high frequency interference such as, for example, noise. If such a low frequency, high amplitude sound signal appears, no significant compression will take place due to the high amplitude level. The noise, for example, is then also fully audible. If, on the other hand, the level of the low frequency sound signal drops, the compander effect will start to become apparent, and this results in the further drawback that the noise is modulated in dependence on the level of the low frequency sound signal.
A known system of this type, commonly termed a Dolby system, as disclosed in German Auslegeschrift [Published patent application] No. 14 87 276, is constituted by a compander system having a first signal path for transmitting the useful signal with a linear dynamic characteristic and full bandwidth, and a second signal path for influencing the dynamic characteristic of the useful signal in a certain frequency range, as well as an auxiliary circuit in which the output signals of the two signal paths are combined, the system being arranged to shift the limit frequency of the certain frequency range to higher values with increasing amplitude of the useful signal.
Such an amplitude dependent shift of the limit frequency of that frequency range in which the dynamic regulation takes place suitably serves to substantially prevent the above-mentioned difficulties. In the known circuit, this effect is realized by the use of two series-connected filters, the second filter of which can be controlled with respect to its limit frequency by means of controllable diodes. Two filters, however, involve a relatively large number of components which again makes it more difficult to maintain uniformity in the resulting characteristics when the system is to be mass produced. Moreover, it is not possible with the known circuit to shift the frequency characteristic while maintaining it exactly parallel to its original position, although this would be desirable to overcome the above-mentioned difficulties.